Video Transcript
Assume that in flies, body color and the size of wings are determined by two different genes, as outlined in the table provided. Body color alleles are capital G for gray and lowercase g for black. Wing size alleles are capital N for normal and lowercase n for small or vestigial. In a fly that has the genotype lowercase g lowercase g capital N capital N, what combination of alleles can be produced in the gametes? (A) Capital G capital N only, (B) lowercase g capital N only, (C) capital G capital N and lowercase g capital N, or (D) lowercase g lowercase g and capital N capital N.
Let’s start by defining some terms that appear in this question. A gene is a sequence of DNA that contains the information needed to produce a certain characteristic, such as body color or wing size in this example. And an allele is an alternative version of a gene. For example, one version of the body color gene produces a gray color. Another version of this gene produces the black color. These different versions of this gene are these genes alleles.
An individual’s genotype is the combination of alleles that individual has for a given gene or genes. We may remember that most animals are diploid, meaning that they have two copies of each gene, meaning that, for example, for the body color gene, an individual might have two capital G or gray alleles, one inherited from that individual’s mother and the other inherited from that individual’s father. Together, these alleles make up that individual’s genotype.
An individual could have any combination of the available alleles for a gene. So for this body color gene, the possible genotypes include capital G capital G, capital G lowercase g, and lowercase g lowercase g. We may also remember that gametes, that is, sperm and egg, are produced by a special type of cell division called meiosis, where the original diploid cell divides twice so that each of the final gametes produced is haploid, meaning that it has just one copy of each chromosome and therefore one copy of each gene. Another important characteristic of gametes produced through meiosis is that due to the mixture and separation of alleles, each gamete can have a different genetic composition.
Let’s look at how this would work for one gene for an individual heterozygous at gene G and, therefore, with the genotype capital G lowercase g. The original cell that begins meiosis in such an individual would have both alleles capital G and lowercase g. However, during the divisions in meiosis, these alleles would be separated from each other so that each of the resulting gametes would have only one capital G or lowercase g allele.
Now, let’s look at the genotype presented in our question, lowercase g lowercase g capital N capital N. Since we’re dealing with multiple genes, we can use the FOIL technique to ensure that we find all the possible combinations of alleles that might be produced in gametes of this individual. If we combine the first two alleles of each type we see in the genotype, we get lowercase g capital N, which is one possible combination of alleles we might see in a gamete from this individual. If we then combine the first allele of the first gene and the second allele of the second gene or the two outer alleles, we again get lowercase g capital N. We can then combine the two inner alleles to get lowercase g capital N and combine the two last alleles of each type to once again get lowercase g capital N.
In this way, we’ve determined that all of the gametes produced by this fly would have the genotype lowercase g capital N. We can actually tell right away that the answer should not be (D) because each gamete should have only one copy of each gene. So no gametes should have two g alleles or two N alleles, for example. We can also tell right away that the answer should not be (A) or (C) because both of these choices include a capital G allele and the genotype of the individual producing these gametes does not contain a capital G that could potentially be passed on to those gametes.
So that leaves (B), which is also what we found using the FOIL method. In a fly that has the genotype lowercase g lowercase g capital N capital N, the combination of alleles that can be produced in the gametes is lowercase g capital N only.
